The present embodiments relate to wireless communications systems and are more particularly directed to a wireless transceiver that performs signal operations in response in part to timing signals received from the global position satellite system.
Wireless communications have become prevalent in business, personal, and other applications, and as a result the technology for such communications continues to advance in various areas. One such advancement includes the use of spread spectrum communications, including that of code division multiple access (“CDMA”). In such communications, a user station (e.g., a hand held cellular phone) communicates with a base station, where typically the base station corresponds to a “cell.” CDMA systems are characterized by simultaneous transmission of different data signals over a common channel by assigning each signal a unique code. This unique code is matched with a code of a selected user station within the cell to determine the proper recipient of a data signal.
CDMA continues to advance along with corresponding standards that have brought forth a third generation CDMA also referred to as 3G cellular. 3G cellular includes two standards, namely, IS2000 which is Qualcom based and supports IS95 in one operational mode, and a wideband CDMA which is also referred to as WCDMA and which has a 3GPP standard. Communications performed under these standards require a timing reference so as to support encoding and synchronized decoding of the communications. For example, one level of such encoding is the use of signal spreading such as using a Walsh code. As another example, CDMA communications may be encoded through the use of both a long code and a short code. In order to properly encode the communications for transmission, the short and long codes must be properly synchronized to some reference time. In addition, when these communications are received by a user station, the user station synchronizes its operation with respect to the short and long code of the transmitting base station and, thus, the user station also necessarily relies on the proper synchronization of the base station with respect to its short and long code.
Given the need for a timing reference, and in the instance of IS2000 by way of example, one present state of the art base station includes a global position receiver that obtains its timing reference in response to signals from the known global position satellite (“GPS”) system. The GPS system is commonly known to transmit geographic positioning information but such information is not used in the present context; instead, the GPS system is also known to issue a periodic pulse along with a time message every second according to an atomic clock. In CDMA, one state of the art base station uses both the GPS pulse as well as the corresponding time message. The time message is used to initialize a value in a chip count register while the frequency of the pulse provides a reference into a local oscillator within the receiver. Specifically, the oscillator includes a phase locked loop (“PLL”) that locks its frequency in response to the frequency of the GPS pulse. The locked frequency is then used to generate a local master clock signal that is used by the receiver to increment the chip count register. Thus, once the chip count register stores a value in response to the time message, the count is then incremented by the local PLL oscillator.
While the above-described state of the art has been shown to provide an operable base station for purposes of synchronizing the base station and thereby to facilitate synchronized transmissions, the present inventors have observed that such an approach also provided various limitations and drawbacks. For example, because the master clock is locked to the GPS timing by way of a PLL, the undesirable phase noise that is inherent in a PLL setup is introduced into the timing signals. As another drawback, the reduction of such noise requires a sophisticated PLL that is therefore relatively complex to implement and increases cost, and cost increases are themselves highly undesirable and indeed sometimes unacceptable in the continued advancement of the competitive market for cellular devices. As another drawback, the master clock signal from the preceding approach is used to clock various devices as would be expected of a master signal, while in fact only certain receive and transmit functions require the synchronization such as to perform correlation operations. However, as a master clock signal, the PLL-induced noise in that master clock signal also affects other circuits within the receiver and, indeed, those affected circuits often therefore require clock phase corrections. Still other drawbacks and limitations may be observed by one skilled in the art.
In view of the above, there arises a need to provide an approach for an improved wireless transceiver operating in synchronization to a system or other universal time signal, as is achieved by the preferred embodiments discussed below.